1. Technical Field
The present invention relates to a method and apparatus for treating silicon wafer wastes. More particularly, the present invention relates to a method and apparatus for recycling and treating the wastes of silicon wafer cutting and polishing processes so as to add value to products and effectively solve environmental pollution problems.
2. Description of Related Art
Cutting silicon ingots into silicon wafers (chips) of uniform thickness has been a standard process in the semiconductor chip and solar panel industries. The cutting process typically involves a special metal wire saw as the cutting tool and the high-hardness silicon carbide (SiC) as the abrasive. In addition, a mineral oil such as polyethylene glycol (PEG), diethylene glycol (DEG), or propylene glycol (PG) is used as the cutting and cooling fluid. When the metal wire saw moves back and forth at high speed, silicon carbide acts as a medium for enhancing the cutting performance of the wire saw so that silicon chips can be effectively cut. During the cutting process, the PEG-based cutting fluid is introduced to the portion being cut not only for cooling and lubricating purposes, but also to bring the silicon particles and impurities generated by cutting out of the cutting grooves, thereby ensuring a smooth and continuous wafer cutting process and facilitating subsequent procedures.
While silicon wafers are being cut and polished, the cutting fluid introduced to the cutting area is continuously discharged, after use, from the cutting area to carry away the silicon particles, impurities, and ground silicon carbide generated by cutting. Thus, a waste oil slurry of high particle concentration and high turbidity is produced. Traditionally, most of the silicon wafer cutting factories either dispose of the useless oil slurry as waste or burn it with heavy oil and then bury the ashes. In either case, however, a waste treatment company must be hired, which is costly. In addition, incineration of the waste oil slurry generates considerable carbon dioxide and is therefore environmentally unfriendly. Hence, neither incineration nor landfill treatment is an ideal solution to the waste oil slurry generated from the silicon wafer cutting and polishing processes.
Presently, attempts are being made to recycle the cutting oil from the waste oil slurry by means of filtering equipment, in which solid-liquid separation is typically carried out using applicable membrane techniques. This approach, though capable of reclaiming some of the oil for reuse in the silicon wafer cutting and polishing processes, still produces a huge amount of sludge, or better known as “filter cake”. The sludge contains silicon carbide, silicon particles, and PEG that are recyclable and reusable, but is nevertheless buried as waste, raising environmental protection issues too. Moreover, it is cost-ineffective to reclaim the oil simply by solid-liquid separation. This is mainly because the waste oil slurry is filtered through membranes without proper pre-treatment, so the pores of the membranes tend to be clogged. As a result, the operating cycles of the membranes are shortened, and the associated costs increased.
Besides, driven by the recent rigorous development of the solar energy industry, the demand of silicon carbide, which was formerly used as a consumable, greatly outstrips supplies. In consequence, the use of PEG for lubricating and cooling purposes also rises significantly, making PEG expensive. Therefore, in consideration of costs and environmental protection, it is necessary not only to recycle oil from the waste oil slurry, but also to separate and recycle the useful silicon carbide and silicon particles from the filter cake, with a view to reducing the total amount of wastes, minimizing pollution, and lowering waste disposal costs.
Now that no treatment methods so far are capable of reaching a balance between recycling costs and environmental protection while featuring efficient and low-cost operation, the silicon wafer cutting industry is facing higher and higher pollution prevention costs. It is therefore imperative for members of the industry to not only increase product quality but also improve pollution prevention techniques so that both economic and environmental protection goals can be met, production costs lowered, and industrial competitiveness enhanced.